The number of electronric circuits that can be manufactured per unit area of silicon or board space has increased dramatically in recent years. This increase in circuit density has produced a corresponding increase in the number of connections required between the various electronic circuits. Integrated circuit chips and boards require not only high density but high reliability connections to other integrated circuit chips and boards to facilitate the manufacture of more complex products. The higher circuit density mandates the higher connection reliability because the probability of the product failing increases with the rising number of circuit connections within that product. Therefore, in order to maintain the product reliability, the reliability of the individual connections must increase. In addition, the connections must maintain this reliability even though the connections are connected, separated, and reconnected several times during the manufacturing of the product. This connecting and reconnecting process is necessary to test the individual electronic circuit components before they are assembled into a final product.
The density, reliability, and testing requirements of electronic circuit connections have been met with a variety of interconnection techniques. One such technique is to wire bond the terminal connections of several electronic circuits together. This involves the mechanical and thermal compression of a soft metal wire, typically gold, from one circuit to another. Wire bonding does not lend itself to high density or reliability because the wires break and are mechanically difficult to handle as the wires get longer due to making connections to all but the peripheral terminal connections of the electronic circuits. Another technique used for circuit interconnection is to place a bump or ball of soft metal, such as gold or a gold composite, onto the terminal connection of the electronic circuit. The pattern of metal balls (corresponding to the pattern of terminal connections) match a predesigned pattern of terminal connections on another electronic circuit. The two patterns are joined and then heated so that the metal balls adhere to both circuits, thereby forming the connection. This technique enables high density interconnections because the number of connections is only limited by the space required to separate individual metal balls. However, the technique creates testing problems because the connections have to be heated to be disconnected, the balls must be reformed once disconnected, and the balls have to be reheated again when the circuits are reconnected during the test procedure. This is a very expensive procedure which also has a limit on the number of times the metal balls can be heated and reheated before the metal balls must be stripped off and new ones redeposited in their place. The expense and test requirements of this interconnection technique limits its usefulness.
In response to the problems encountered with the wire bonding and metal ball interconnection techniques, a variety of elastomer interconnection materials and techniques have been developed. Generally, the elastomer material is one in which current is conducted through the material by a plurality of conductive paths embedded within the elastomer material. The conductive paths are typically small diameter wires or small area columns of conductive material which are insulated from each other by the elastomer material. Individual wires or columns of conductive material are spaced closely with respect to each other and groups of the closely spaced conductive paths are arranged on a grid or other pattern to match the electrical circuit terminal connection pattern. A typical elastomeric interconnection technique involves placing the elastomeric material between two electronic circuits and compressing the material between the circuits. Once compressed, the electrical connections between the two circuits are established because the small spacing between individual conductive paths and the high number of paths used for a single contact assures contact is made to an electronic circuit connection terminal. The groups of conductive paths are arranged within the elastomeric material in array or other patterns which align to the pattern of connection terminals on the electronic circuit.
The elastomeric interconnection technique is especially useful in testing electronic circuits because circuits can be removed and replaced and the elastomeric interconnection material will expand and recompress without damaging either the electronic circuits or the material itself. This is a very inexpensive test method because the interconnection material may be reused and no additional processing is required to repair damaged circuits. However, a basic problem with this method of interconnecting electronic circuits is the reliability of the connections over a large array of interconnections. This is a problem because the elastomeric connector material has a resilience which is not uniform across a large area. The nonuniform resilience results in two problems. First, the elastomeric material places different mechanical strains on different parts of the electronic circuit in response to the compressive force holding the circuits together. Second, the differences in resiliency lead to electrical interconnections which have either high contact resistance or are open altogether.
The effects of nonuniform stress are especially evident when testing involves protracted time periods under extreme environmental conditions, as would be the case for burn-in testing. Therefore, many of the advantages associated with the elastomeric interconnection material, such as reliability and inexpense of non-damaged circuits, are nonexistent when large area connections are under compression for extended periods of time. Attempts to minimize these effects have typically centered around using bent or corrugated metal wires in the elastomeric material. The pre-bent wires compress in a more uniform manner and as such decrease nonuniform stress across the elastomeric material. However, because the wires are pre-bent, they are difficult to align with respect to each other in the elastomeric material. This results in more nonuniformity in the compression of the material over large areas because the wires not aligned with respect to each other retard resilience. The resilience of this material cannot be controlled because of the lack of control over bent wire alignment, and therefore, the compression of the elastomeric material will be nonuniform over large areas.